1. Field of the Invention
This invention relates to amplifiers in general; and, in particular, to preamplifiers for read heads in magnetic disk drives.
2. Discussion of the Related Art
The preamplifier in this application processes a differential signal from a magneto-resistive read head for a disk drive. In addition to signal amplification, the preamplifier contributes noise. If the preamplifier has a high gain, noise introduced by subsequent stages can be rendered, as compared to noise introduced by the preamplifier, relatively insignificant. Thus, noise in a preamplifier should be minimized, while the preamplifier must also maximize gain and bandwidth.
FIG. 1 shows a prior art preamplifier 100 receiving a differential signal across terminals 110 and 111, which are amplified by differential stages 100a and 100b. Differential stages 100a and 100b include emitter-coupled transistor pairs 101 and 102, and 105 and 106. Differential stages 100a and 100b are used because of the high gain required. For example, a bias of several hundred millivolts in the differential signal across terminals 110 and 111 (i.e., the signal provided by the read head) can exceed the rail-to-rail voltage. However, preamplifier 100 of FIG. 1 is undesirable because of the noise in four transistors.
As shown in FIG. 1, each of differential stages 100a and 100b is used as a single-ended amplifier. The signal at terminal 110 is coupled to the base terminal of transistor 101 and the signal at terminal 111 is coupled to base terminal of transistor 105. Current sources 113 and 114 provide bias currents to differential stages 100a and 100b, respectively. Transistors 115-118, which are each biased by a common reference voltage source 119, form cascodes to provide an AC ground. Differential amplifiers 120 and 121 drive differential stages 100a and 100b respectively to provide an output differential voltage at terminals 122 and 123. Capacitors 124 and 125 filters out high frequency AC noise signals in input terminals 110 and 111, respectively. Resistors 126-129 are typically given the same resistance.
Preamplifier 100's gain can be set by selecting a suitable resistance value for resistors 126-129. Roughly, the gain is determined by the ratio of this selected resistance to the emitter resistance in each of transistors 101, 102, 105 and 106. Gain is increased by a higher resistance or a larger current in each of transistors 115-118.
Since differential stages 100a and 100b are essentially two single-ended amplifiers, preamplifier 100 does not have a good common mode rejection characteristics.
FIG. 2 shows a preamplifier 150 in the prior art, which is a variation of preamplifier 100, for use with multiple read heads. (To simplify discussion, like elements in the figures of the application are provided like reference numerals.) In preamplifier 150, multiple differential signals from multiple read heads share differential stages 150a and 150b. For example, as shown in FIG. 2, the differential output signal from a first read head is provided at terminals 110 and 111 and coupled into differential stages 150a and 150b via the base terminals of transistors 101 and 105, respectively. In like manner, the differential output signal of a second read head is provided at terminals 151 and 152 and coupled into differential stages 150a and 150b via the base terminals of transistors 153 and 154, respectively. A mechanism (not shown) is provided to select which one of the output differential signals from the multiple read head is coupled into preamplifier 150.
To minimize shot noise, transistors 101, 102, 115 and 116 are made as large as practical. At the same time, however, without reducing the resistances in resistors 126-129, increasing the currents in transistors 101, 102, 115 and 116 limits the dynamic range of the output signals. Reducing the resistances of resistors 126-129 reduces the gain of the preamplifier.